Contrast-enhancing picture tube faceplate and process for producing same

ABSTRACT

Reflectivity of the faceplate of a color television receiver picture tube is reduced in order to achieve improved contrast in displayed images with minimal decrease in brightness and without objectionable halation of the displayed image by blasting the inner surface of the faceplate with an air-propelled slurry of abrasive particles having substantially uniform size in the range of 5-16 microns in diameter, preferably 9.5 microns. The abrasive-blasted surface of the faceplate is then etched in hydrofluoric acid. The faceplate thus processed exhibits an inner surface pitted with craters having an average diameter of approximately 5 microns and depths ranging up to approximately 1.5 microns at a density of substantially 107 craters per square centimeter.

ilnited States atent 1 Scott et al.

[ CONTRAST-ENHANCING PICTURE TUBE FACEPLATE AND PROCESS FOR PRODUCING SAME [75] Inventors: Stanley J. Scott, Buffalo; William J. Noroski, North Syracuse; Wilfred H). .Rublack, Liverpool, all of NY.

[731 Assignee: General Electric Company,

Syracuse, NY.

[22] Filed: I Aug. 2, 1972 [21] Appl. No.: 277,398

3,616,098 10/1971 Falls 313/92 R X [1 1 3,813,568 [451 May 28, 1974 Primary Examiner-Herman Karl Saalbach Assistant Examiner-Siegfried H. Grimm Attorney, Agent, or Firm-Marvin Synder 5 7 ABSTRACT Reflectivity of the faceplate of a color television receiver picture tube is reduced in order to achieve improved contrast in displayed images with minimal decrease in brightness and without objectionable halation of the displayed image by blasting the inner surface of the faceplate with an air-propelled slurry of abrasive particles having substantially uniform size in the range of 5-16 microns in diameter, preferably 9.5 microns. The abrasive-blasted surface of the faceplate is then etched in hydrofluoric'acid. The faceplate thus processed exhibits an inner surface pitted with craters having an average diameter of approximately 5 microns and depths ranging up to approximately 1.5 microns at a density of substantially l0 craters per square centimeter.

3 Claims, 14 Drawing Figures PATENTEDMAY 28 m4 SHEET 1 BF 6 FIG.2

- PRIOR ART FIG. I

PRIOR ART FIGS LARGE CRATERS 0 Q0 REFLECTIVITY we) E s A E R C E D 5 E \1) W B E E Z S R| c m B AS 2 E NC H A m m 0 M N S l O O. 7. 6 5

PATENTEDHAY 28 m4 SHEET 2 BF 6 FlG.4

FIG.9

SJSlSBGB PATENTEDHAY 28 m4 SHEET 3 BF 6 PATENTEBHAY28 1914 3,813,668

SHEEI 1, OF 6 PATENTEI] m 28 i974 331356 8 sum 5 OF 6 1 CONTRAST-ENHANCING PICE TUBE FACEPLATE AND PROKIESS FOR PRODUCING SAME INTRODUCTION This invention relates to color television receiver picture tubes, and more particularly to picture tube faceplates having an inner surface of predetermined roughness in order to enhance contrast, and a process for achieving such surface roughness.

ln color television picture tubes of conventional manufacture, the faceplate transmission is held to as high a value as possible. This permits the image displayed through the faceplate to be of high brightness. However, the faceplate also exhibits high reflectivity due to its essentially smooth surfaces. As a result, the viewer sees a glare on the faceplate caused by light sources whose optical radiation is reflected by, the faceplate. Additionally, reflection of ambient light from the entire phosphor screen-glass interface itself causes diminished contrast and diminished saturation inthe displayed colors. v

In the past, picture tube faceplates containing scratches, surface discoloration, and other minor surface blemishes on their inner surfaces have been reclaimed for use by spraying or blasting the blemished inner surface of the faceplate with an air-propelled slurry of abrasive particles such as aluminum oxide of substantially 22 or 23 microns in diameter and'then etching the abrasive-blasted surface. As a result, the surface becomes pitted with craters which, if of sufficient depth, and formed in sufficient density per unit of surface area, can obliterate minor surface blemishes. This process has thereby been successfully employed to remove such blemishes by changing the overall surface texture from smooth rough; that is, by pitting the surface with craters. The image displayed on such tube, however, exhibits a haze, or halation, accompanied by a reduction in brightness. The halation is believed to be caused by refraction due to existence of the craters which introduce various angles of glass surface through which light emitted by the phosphors passes. Beneficial effects that result from this process, however, include an increase in contrast .due to reduced reflectivity of the roughened surface of the faceplate, and a decrease in area of each visible reflection of an external light source on the faceplate for the same reason, without any adverse effects on resolution.

The present invention contemplates an improved picture tube which exhibits the beneficial effects obtained through use of an abrasive-blasted and etched faceplate. but with substantially no halation and with less brightness loss. This is achieved by blasting the inner surface of the faceplate with an air-propelled slurry of aluminum oxide abrasive particles of substantially uniform diameter in therange of 5-1 6 microns, preferably 9.5 microns, and then etching the abrasive-blasted surface of the faceplate in percent hydrofluoric acid for a predetermined time. The resulting etched surface of the faceplate contains a higher density of craters per unit area than the aforementioned reclaimed tubes. These craters are of smaller average diameter and smaller average depth than the craters resulting from the reclamation process described, supra. The lack of halation brought about by this process is believed due to the fine grain of the scattering surface which more closely approaches auniformly smooth, frosted surface leaving few relatively large areas of glass surface at angles skewed to the plane of the glass to produce refraction. The greater brightness brought about by this process is believed due to greater direct optical contact between the phosphors and the faceplate since the phosphor material essentially bridges over the tops of the craters, leaving smaller gaps between the phosphor material and the glass where the craters are smaller, than where the craters are larger; that is, where the gaps between phosphor material and glass are small, there is greater direct optical contact therebetween than where the gaps are largenMost of the light emitted by the phosphor material that enters the faceplate glass in a direction substantially perpendicular to the plane of the faceplate is that furnished by the phosphor material where it is in direct contact with the faceplate. This greater direct optical contact, in the case where the gaps are smaller, allows more light to enter the glass in a direction substantiallyperpendicular to the plane of the faceplate, thereby enhancing opticalefficiency and brightness by reducing entry of light into the glass at angles other than orthogonal to the plane of the'glass.

Accordingly, one object of the invention is to provide a process for roughening the inner surface of television picture tube faceplates to provide improved image contrast with but slightly reduced image brightness.

Another object is to provide a process for reducing reflectivity of television picture tube faceplates without introducing objectionable halation of the image.

Another object is to provide a television picture tube faceplate exhibiting improved contrast in displayed images with but slightly reduced image brightness.

Another object is to provide a television picture tube faceplateexhibiting low reflectivity without introducing objectionable halation in the displayed image. I Briefly, in accordance witha preferred embodiment of the invention, a process for producing an improved television picture tube faceplate comprises subjecting the inner surface of a faceplate to an air-propelled slurry of aluminum oxide abrasive particles of substantially uniform diameter in the range of 5-l 6 microns in diameter to form checks therein by abrasion, and etching the abraded inner surface of the faceplate in 20 percent hydrofluoric acid solution for a predetermined time.

In accordance with another preferred embodiment of the invention, an improved television picture tube faceplate comprises a'glass panel having an inner surface and an outer surface. The inner surface is pitted with craters at an average density of substantially l0 craters per square centimeter, so as to exhibit lower reflectivity than the outer surface. Average crater diameter is approximately 5 microns, while crater depths range up to approximately 1.5 microns.

BRIEF DESCRIPTION OF THE DRAWINGS The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself, however, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a cross-sectional view of a portion of a conventional color picture tube faceplate showing the phosphor dots deposited thereon;

FIG. 2 is a cross-sectional view of a portion of the faceplate of a television picture tube which has been reclaimed by the known process of blasting with relatively large diameter abrasive particles followed by an acid etch;

FIG. 3 is a cross-sectional view of a portion of a television picture tube faceplate which has been roughened in the manner of the invention described herein;

FIG. 4 is an electron micrograph of untreated glass conventionally employed for picture tube faceplates;

FIGS. 5A and 5B are electron micrographs of picture tube faceplate glass blasted with abrasive particles of 22.5 microns diameter and thereafter acid etched;

FIGS. 6A and 6B are electron micrographs of picture tube faceplate glass blasted with 9.5 micron diameter abrasive particles and thereafter acid etched;

FIG. 7 is an electron micrograph with so-called Y Modulation" of picture tube faceplate glass blasted with abrasive particles of 22.5 microns diameter and thereafter acid etched;

FIG. 8 is an electron micrograph with so-called Y Modulation of picture tube faceplate glass blasted with abrasive particles of 9.5 microns diameter and thereafter acid etched;

FIG. 9 is an electron micrograph of a cross-section of picture tube faceplate glass blasted with abrasive particles of 22.5 microns diameter and thereafter acid etched;

FIG. 10 is an electron micrograph of a cross-section of picture tube faceplate glass blasted with abrastive particles of 9.5 microns diameter and thereafter acid etched;

FIG. 11 is an electron micrograph of a portion of the glass illustrated in FIG. 10, but at twice the magnification; and

FIG. 12 is a curve of phosphor screen efficiency vs. reflectivity, for 84.5 percent transmittance glass, used in explaining the instant invention.

DESCRIPTION OF TYPICAL EMBODIMENTS FIG. I depicts a cross-section of a typical faceplate I0 of a conventional picture tube, such as that of the well-known aperture mask type tube. The faceplate may typically be comprised of 69 percent transmittance glass; that is, only 69 percent of light entering the faceplate glass emerges therefrom at the opposite side, the remaining 31 percent being absorbed by the glass. Thus, when any one of phosphor dots l2v deposited on the substantially smooth inner surface 14 of the faceplate emits a predetermined color under electron beam excitation, whether that color be red, green or blue, only about 69 percent of the light impinging on phosphor-glass interface 16 from the emittingphosphor dot at an angle of incidence less than the'critical angle emerges from front surface 18 of faceplate 10.

Although glass of transmittance much higher than 69 percent is readily available, it cannot be used in tubes having faceplates of the type depicted in FIG. 1 without a substantial, undesirable sacrifice in image contrast under high ambient lighting viewing conditions. This is because ambient light is reflected at the phosphor-glass interface 16. This reflected ambient light tends to swamp out the colored light produced by the phosphors, thereby degrading contrast of the image. Since the ambient light within the faceplate glass is made up of both incident and reflected components, each of which passes through the entire faceplate thickness in order to obscure the image being viewed, luminance of the' reflected ambient is reduced by a factor equal to the square of the transmittance-value, while luminance of the displayed image, produced by light passing through the faceplate only once, is reduced by a factor equal to the transmittance value. By thus using glass of only 69 percent transmittance, contrast is enhanced at the expense of display brightness. Enhancement of contrast in this manner, however, is at best not entirely satisfactory since phosphors presently available cannot provide sufficient brightness to overcome the high loss due to absorption in the faceplate glass and still present a bright-appearing image under conditions other than low ambient lighting levels.

It is well known that shallow blemishes or defects of various types, such as scratches, discolorations, etc., on the inner surface of the faceplate, which degrade quality of the displayed image, may be eradicated by blasting the blemished surface of the faceplate with an airpropelled slurry of abrasive particles, such as aluminum oxide,-and then etching the abrasive-blasted surface. However, in order to achieve this result, relatively large 'size abrasive particles, substantially 22 or 23 microns in diameter, have been employed. Use of the large size abrasive particles insures that sufficient surface erosion occurs to completely obliterate the blemishes. Thus the inner surface of the faceplate becomes pitted with craters that typically range up to approximately 4 microns in depth and exhibit an average diameter of approximately 15 microns. These craters may be formed at a density of about 3.5 X 10 craters per square centimeter. As a result, there is sufficient cratering per unit area, and'to a sufficient depth, to remove completely that portion of the inner surface in which the shallow blemish or defect previously existed. Additionally, the entire inner surface of the faceplate is rendered more uniform. This condition is depicted in FIG. 2, wherein like numerals correspond to those of FIG. I, and wherein inner surface 14 and phosphor-glass interface 16 can be seento be quite rough in comparison with untreated inner surface 14 and phosphor-glass interface 16 in the apparatus of FIG. I.

When a tube having a faceplate of the configuration shown in FIG. 2 is operated, the blemishes or defects that had been previously removed can no longer have any effect on the image, resulting in display of improved images. However, a new problem presents itself as a result of roughened inner surface 14 of faceplate 10. Because of the relatively large area of skewed inner surface 14, or inner surface that is nonparallel to external surface 18 of the glass, light from phosphor islands 12 enters the glass not only in a direction orthogonal to the plane of external surface 18, but also at various other angles with respect to the plane of surface 18. This is especially true due to the different indices of refraction of the glass and of the gaps 15 between phosphor material 12 and glass 10, which cause bending of the light rays as they pass from one transmission medium to the next. As a result, some light from each one of energized phosphor dots 12 (i.e., the dots struck by the electron beam) is scattered within the glass of faceplate 10. This scattered light, while of lower intensity than that of the light emitted normal to the plane of surface 18 by phosphor dots 12, is nevertheless of sufficient amplitude to produce noticeable halation of the image. The image consequently appears less distinct than that produced by faceplates of the type shown in FIG. I. Additionally, due to the loss of scatter light, the tube exhibits reduced luminance or brightness. Despite the halation, however, the roughened inner surface 14 of the faceplate shown in FIG. 2 is less reflective than the smooth inner surface of the faceplate shown in FIG. I. As a result, less ambient Iightis reflected from surface 14 of the faceplate shown in FIG. 2 than from surface 14 of the faceplate'shown in FIG. 1, allowing the tube employing the faceplate of FIG. 2 to be operated at lower visible brightness levels than the tube employing the faceplate of FIG. I, for any given level of ambient light, to achieve the same visible degree of contrast in the displayed image.

The faceplate of FIG. 3 represents an improvement over the faceplate of FIG. 2 in that reflectivity of'inner surface 14 remains low and yet any halation in the displayed image is markedly reduced. This is achieved by forming surface 14 in a more finely roughened fashion than surface 14 in the configuration of FIG. 2. Consequently, there is less scattering of emitted light through the faceplate due, apparently, to the shallower depressions formed in the glass at the phosphor-glass interface in the faceplate of FIG. 3 than there are in the faceplate of FIG. 2. These shallowed depressions allow greater direct optical contact between-the faceplate glass and the phosphor material inasmuch as the gaps therebetween, due to their smaller size, are essentially filled with phosphor material. Additionally, the sides of the craters formed in the glass are shallower, so that less skewed glass surface is presented to the phosphor material at each gap. Moreover, any gaps that do exist between the phosphor material and the glass are so shallow that very little displacement of light rays, due to refraction, occurs before the light rays enter the glass. Nevertheless, as with the faceplate of FIG. 2, and despite the decrease in haze or halation over the faceplate configuration of FIG. 2, a more uniform-appearing surface is presented to the viewer across the entire area of the faceplate. Additionally, because of the frosted or roughened internal surface of the faceplate, the sparkle of the aluminum coating that is typically deposited over the internal surface of the faceplate atop the phosphor islands is reduced, inasmuch as the reflection of ambient light by the coating in the regions between the phosphor islands is directed in various directions, thereby reducing intensity of reflection in any one direction. Thus, when the tube is not energized, it exhibits a uniformly gray appearance even when a bright source of light is directed onto the faceplate.

FIG. 4 is an electron micrograph, at magnification power of 1,200, of the inner surface of a relatively smooth faceplate (which typically contains a low density, or stippling, of 20 micron diameter impressions) supplied by a manufacturer of picture tube faceplates. In conventional picture tube fabrication processes, the texture of the inner surface of the faceplate as supplied by the manufacturer is not disturbed, but is retained in essentially unaltered form even in the completed picture tube.

In the electron micrographs of FIGS. 5A and 5B, which are views of the inner surface 14 of the faceplate glass in the configuration of FIG. 2 at magnification powers of 1,000 and 2,000 respectively, there are craters formed in the surface at an average density on the order of 3.5 X 10 craters per square centimeter, with average crater depth ranging up to approximately 4 microns and average crater diameter of approximately 15 microns. These craters were formed by directing an airpropelled slurry of aluminum oxide particles of approximately'22.5 microns in diameter at the inner surface of the faceplate, and then etching the inner surface in hydrofluoric acid. By way of comparison, in the electron micrographs of FIGS. 6A and 6B, which are views of the inner surface 14 of glass faceplate 10 in the configuration of FIG. 3 at magnification powers of 1,000 and 2,000 respectively, there are craters formed in the surface at an average density on the order of 10 craters per square centimeter, with average crater depth rang ing up to approximately 1.5 microns and average crater diameter of approximately 5 microns. These, craters were formed by directing an air-propelled slurry of aluminum oxide particles of approximately 9.5 microns in diameter at the inner surface of the faceplate, and then etching the inner surface in hydrofluoric acid. Thus it is clear that bombardment with larger size particles produces large diameter craters and, for a given volume of aluminum oxide material, less craters are formed using the larger diameter particles.

FIGS. 7 and 8 are electron micrographs, at magnifications of 2,000 of the inner surface of picture tube faceplates of the type shown in FIGS. 2 and 3, respectively. These micrographs are made by the so-called Y Modulation technique wherein generally horizontal lines are traced electronically across the image of the surface to emphasize the three dimensional character of the image. These lines were produced on a Cambridge Scanning Electron Microscope, model S-4, sold by Kent-Cambridge Company of Cambridge, England. A detailed description of the Y Modulation technique may be found in Scanning Electron Microscopy by Patrick R. Thornton, published by Barnes and Noble, I968. It is clear, from FIGS. 7 and 8, that craters formed on the inner surface of the picture tube faceplate are smaller, on the average, and of greater density per unit area when the abrasive particle size is 9.5 microns than when the particle size is 22.5 microns.

FIGS. 9 and 10 are electron micrographs, at magnifications of 1,000, of a cross-section of picture tube faceplate glass of the type shown in FIGS. 2 and 3, respectively. These electron micrographs confirm the Y Modulation electron micrographs of FIGS. 7 and 8, respectively, in that they show craters of shallower depth and smaller diameter in the faceplate glass of FIG. 10 than in the faceplate glass of FIG. 9. FIG. 11

is an electron micrograph showing the left-hand por-' tion of FIG. I0 at magnification power of 2,000, to illustrate the craters in greater detail.

FIG. 12 illustrates the effect of varying reflectivity of a picture tube faceplate of 84.5 percent transmittance glass by blasting the inner surface of the glass with an air-propelled slurry of abrasive particles and then etching the abrasive-blasted glass to form craters by erosion. As evidenced from point A on the curve of FIG. 12, reflectivity of a screened faceplate made with smooth glass surfaces is approximately 44 percent while phosphor efficiency at that reflectivity is typically about 6.5. Phosphor efficiency is herein defined as foot lamberts per milliwatt per square inch of power density input corrected for the faceplate transmission (i.e. assuming an absolutely clear faceplate). If the inner surface of the glass is cratered by abrasive blasting with I 9.5 micron diameter abrasive particles followed by a single etch so as to produce a 20 percent reduction in reflectivity from the smooth condition, as indicated by point B on the curve, phosphor efficiency drops only by about 5 percent. Due to the shape of the curve, however, a greater reduction in reflectivity, as produced by employing abrasive particles of 22.5 microns diameter, brings on a disproportionately larger decrease in phosphor screen efficiency.

In fabricating faceplates of the type shown in FIGS. 3, 6A and 6B, 8, l and 11, the inner surface ofa relatively smooth faceplate of the type shown in FIG. 4, supplied by a .typical manufacturer of faceplates such as Corning Glass Works of Corning, New York, is blasted with aluminum oxide particles of -16 microns in diameter, preferably 9.5 microns in diameter. The abrasive particles are typically suspended in the form of a slurry, which is propelled by air pressure against the inner surface of the faceplate. This abrasion process, which results in formation of checks in the glass over the entire surface area exposed to the abrasive, is typically performed on an abrasive blasting machine, such as those sold by Vapor Blast Manufacturing Company, Milwaukee, Wis., which employs five nozzles to blast the slurry onto the inner surface of a 25-inch picture tube faceplate for 42 seconds while the nozzles oscillate in one dimension at the rate of one-half cycle per second. After abrasive blasting is completed, the faceplate is removed from the abrasive blasting machine and its inner surface is etched for 30 to 45 seconds, typically 35 seconds, in percent solution of hydrofluoric acid at room temperature (i.e., approximately 72F) to enlarge the checks into minute craters and thereby achieve a low reflectivity, frosted finish on the inside of the faceplate. Upon completion of the etch, the faceplate is rinsed in water.

The foregoing describes a picture tube faceplate having enhanced contrast with but minimal decrease in brightness achieved through use of a roughened inner surface. The inner surface of the faceplate is roughened to reduce reflectivity without introducing visible halation of the image by blasting the inner surface of the faceplate with an air-propelled slurry of abrasive particles of substantially uniform diameter, preferably 9.5 microns, and then etching the abrasive-blasted surface of the faceplate in hydrofluoric acid solution. The resulting etched surface of the faceplate exhibits a frosted appearance and contains a relatively high density of fine grained craters which bring about reduced reflectivity without causing excessive scattering of light within the faceplate.

While only certain preferred features of the invention have been shown by way of illustration, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.

We claim:

1. An improved television picture tube faceplate comprising: i

a glass panel having an inner surface and an outer surface, said inner surface being pitted with craters at a substantially uniform, average density of approximately 10 craters per square centimeter to exhibit lower reflectivity for any glass transmittance without objectionable halation and without appreciable loss in brightness, said craters having depths ranging up to approximately 1.5 microns; and

a plurality of regions of phosphor material coated on said inner surface, said phosphor material emitting light when undergoing electron bombardment.

2. A television tube exhibiting improved capability to display images of high contrast, said tube including a glass faceplate having a finely roughened inner surface pitted with craters at a substantially uniform, average density of approximately 10' craters per square centimeter, said craters having depths ranging up to approximately 1.5 microns, and a relatively smooth outer surface, said inner surface having a phosphor screen coated thereon and exhibiting, at the interface of said screen and said inner surface, lower reflectivity for any given glass transmittance without objectionable halation and without appreciable loss in brightness, each region of phosphor making contact with substantially the entire inner surface of said glass beneath said'region, said phosphor material emitting light when undergoing bombardment by electrons generated with said tube.

3. The television picture tube of claim 2 whereinsaid craters have an average diameter of approximately 5 microns. 

2. A television tube exhibiting improved capability to display images of high contrast, said tube including a glass faceplate having a finely roughened inner surface pitted with craters at a substantially uniform, average density of approximately 107 craters per square centimeter, said craters having depths ranging up to approximately 1.5 microns, and a relatively smooth outer surface, said inner surface having a phosphor screen coated thereon and exhibiting, at the interface of said screen and said inner surface, lower reflectivity for any given glass transmittance without objectionable halation and without appreciable loss in brightness, each region of phosphor making contact with substantially the entire inner surface of said glass beneath said region, said phosphor material emitting light when undergoing bombardment by electrons generated with said tube.
 3. The television picture tube of claim 2 wherein said craters have an average diameter of approximately 5 microns. 